The present invention relates generally to plastic bottle formation and, more particularly, to a spray coating apparatus for applying first a primer coat and then a top coat of liquid coating material to the interior surface of a plastic bottle to provide a barrier layer between the contents of the bottle and the plastic material used to form the bottle and to limit diffusion of gases through the walls of the bottle.
Containers constructed from thermoplastic material have become widely used in the beverage industry, competing with traditional metal and glass containers. Attractive features of plastic containers include high resistance to rupture, transparency and light weight. A drawback of thermoplastic beverage bottles has been the relatively high cost of the plastic materials which are presently used to produce such bottles. Polystyrene, which costs significantly less than most plastics presently used to form bottles, is readily available as a by-product from the formation of more expensive plastic materials. However, there are presently no polystyrene containers in commercial use as containers for pressurized beverages because of a number of production related problems. Polystyrene bottles to be economically feasible must be capable of being produced rapidly and, as a finished product must be able to contain carbonated beverages under pressure without significant loss of carbonation from the beverage or without diffusion of oxygen from the atmosphere into the beverage. The molecular structure of polystyrene is such that for wall thicknesses that are sufficiently thin to be economically feasible, a coating layer of another material must be applied over the inner surface of the bottle to limit gas diffusion to acceptable levels.
Additionally, it has been found that a coating layer of chemically inert material is necessary to isolate the bottle contents from physical contact with the polystyrene to prevent chemical reaction therewith and deterioration of the taste of the bottled beverage. The most efficient manner presently known by applicant to provide a barrier between the bottled contents and the polystyrene material forming the bottle is to apply first a coating layer of latex primer material and, after the primer layer has dried, to apply a top coat layer of polyvinylidene chloride latex (PVDC latex).
To be economically feasible, it is necessary to apply the coating layers to plastic bottles at relatively high speeds, but in a manner that ensures complete coating of the bottle interior surface and which ensures that the coating layers are within predetermined thickness tolerences. If the coating layers are too thick, then drying time and thus bottle production time, is increased driving up production costs. On the other hand, if the coating layers are too thin or incomplete the intended barrier properties of the coating layers are not realized.
A significant problem in applying precise and even thickness coating layers is caused by the relatively restricted opening and the varying diameter of the interior cavity of most bottles. A further problem with coating application may result from the physical characteristics of the coating medium. The coating materials may be applied as liquid emulsions which are nonnewtonian fluids. Such liquids are not easily dispensed in uniform spray patterns over distances of more than a few centimeters.
The present invention comprises an apparatus capable of applying a precise, uniform coating layer to the interior surface of a bottle having a relatively restricted opening into its interior cavity. The apparatus is adapted to be operational at relatively high speeds even when the coating material being dispensed is a nonnewtonian fluid.